In the field of printing of images, there often arises a need to produce an image on relatively large receivers. Printing of images on large receivers presents various problems. The nature of the problems is often a function of the type of printing being performed. For example, in the case of thermal printing, there is a practical limitation on the size of a thermal printing head. When an image is to be formed on a receiver that is wider than a thermal printing head, a multiple head printing machine is typically used. Two or more printing heads are used to produce overlapping sub-images. The printing heads are coordinated with each other so that each head produces only a portion of the image in an overlapping region The coordination of the printing heads is a difficult and complex task. See, for example U.S. Pat. No. 4,977,410 (H. Onuki et al.), issued Dec. 11, 1990. If not done properly, the overlapping region of the image has objectionable distortions.
Even when great care is used to coordinate the operation of the multiple printing heads, there is still a visually discernible discontinuity formed in the image at boundaries of the overlapping region of the image
Another problem arises when it is desired to produce large images by thermal printing There is a practical limit on the size of a dye-donor film segment that can be used to provide dye for the image. Heretofore, image size has been limited to the size of a dye-donor film patch used to produce the image.
Dye-donor films must be produced with an extremely uniform coating of dye on a carrier film. In order to achieve this high level of uniformity, the dye-donor film is typically produced on a high precision gravuring press. In a gravuring press, the carrier film is coated with repeating sets of dye patches of various colors. Typically, a first gravuring roller coats the carrier film with a patch of yellow dye. A second gravuring roller coats the carrier film with a patch of magenta dye. A third gravuring roller coats the carrier film with a patch of cyan dye. In some instances, a set of patches will include a black dye patch and possibly a special clear overcoat patch or a white undercoat patch. Each of the patches has a limited length that is dictated by the circumference of the gravuring rollers and the number of patches produced in each set. For example, assume that it is desired to make a three color dye patch set with a gravuring press having rollers with a 36 inch circumference. The 36 inch circumference will need to be divided into three segments with a maximum circumferential length of 12 inches. Thus each dye patch produced by such a roller has a maximum length of 12 inches.
In order to produce patches which are longer, larger rollers are required. This requires that a gravuring press must be built especially for the longer length. High precision gravuring presses are extremely complex and expensive to build. A typical high precision gravuring press may cost as much as $100 million. Therefore, as a practical matter, prior art thermal printers are limited to using dye-donor films with patches that are no longer than those available from presently available gravuring presses.
It has heretofore not been possible to produce a thermally printed image that is longer than a patch of dye on the dye-donor film being used to produce the image. Because of limitations dictated by presently available gravuring presses, there has been a practical limitation on the length of an image that is producible in a thermal printing operation.
It is desirable therefore to produce large images that are free of visually discernible distortions and which can be produced with dye-donor films that are generated on presently available gravuring presses.